Until the development by Surface Combustion, Inc. (the assignee of the present invention) of the Unidraw.TM. and the Vacudraw.TM. furnaces described, respectively, in the '091 and '782 Background Patents, furnace atmosphere heat transfer with the work (principally for cooling) was achieved in heat treat furnaces by complicated baffle arrangements. Whether the baffles were moveable or not, such arrangements produce, at best, heat transfer patterns with the work which are sensitive and difficult to accurately control. Examples of prior art attempts to develop improved heat transfer arrangements with improved wind mass flows about the work can be seen in Jomain U.S. Pat. No. 4,836,776 and Moller U.S. Pat. No. 4,906,182. The inventive concepts disclosed however in the '091 and '782 Background Patents, however, utilize a very simple recirculating wind mass scheme which obviates the needs of baffles, movable injection ports, two fan systems, etc. in a simple, low cost furnace construction. The present invention can be viewed as an improvement to the inventions disclosed in the '091 and '782 Background Patents, and one that is especially suited for high temperature applications.
In high temperature applications where the work is heated above 1200.degree. F., typically in the range of 1900.degree.-2050.degree. F., coolant such as water cannot be initially introduced into a "hot" cooling coil situated in the furnace without causing thermal shock and failure of the cooling coil. Thus, in those high temperature applications where the work has to be quickly cooled initially from a high temperature, the VacuDraw furnace had difficulty because a cool gas had to be initially used. Furthermore, several heat treat processes such as marquenching or austempering require that the quench be interrupted after initial cooling to relieve grain size, prevent part deformation etc. Performing interrupted quenching in the prior art arrangements was difficult to achieve.